The invention relates to the lowering of the number of carbamate groups in products made of cellulose carbamate. As is well known, cellulose carbamate is a compound formed when cellulose is heated at elevated temperature in the presence of urea. Cellulose carbamate is an alkali-soluble compound, and this permits it to be converted into the shape of various products by preparing an alkaline solution thereof and then precipitating the products from the solution in desired form, for example, as fibers, films, spongy products, etc.
One of the factors which influences the properties of cellulose carbamate products, and consequently the use thereof, is the carbamate content, that is the number of carbamate groups in the cellulose chains. It is known that carbamate groups to a certain extent increases the sensitivity of the products to water and as a consequence has an effect on the wet strength of the products.
It is possible by reducing the number of carbamate groups to influence the wet strength properties of the products to a desired degree. U.S. Pat. No. 4,583,984 discloses a process for lowering the quantity of carbamate by treating the products with alkali solutions. This procedure in principle makes it possible to completely eliminate the carbamate groups, whereby products consisting of regenerated cellulose are obtained.
It has been found, however, that the lowering of the quantity of carbamate groups with the aid of alkali has an adverse effect on certain strength characteristics, particularly in the case of industrial scale wherein maximum fast and efficient removal of carbamate groups is aimed at, for example, using concentrated alkali solutions or high treatment temperatures.